In general, the functions that solid electrolytes have are largely classified into three functions, electromotive function due to an activity difference, selective ion transmission function and uneven ion distribution function. Referring to the electromotive function out of these functions, a reference substance having known activity and a substance to be measured are allowed to be in contact with one side and the opposite side of a plate-like solid electrolyte respectively. The activity of the substance to be measured can then be determined from an electromotive force value produced therebetween.
In recent years, rapidly increasing attention is paid to ceramic sensors making use of solid electrolytes, and such sensors are now used in large fields. In particular, oxygen sensors making use of zirconia ceramics are of sharp responsiveness and are stable in the electromotive force, so that a partial oxygen pressure can be detected over an extensive range. Besides, those oxygen sensors have the advantage that they can stand up to high temperatures. For those reasons, they are now used in various fields in which energy saving, prevention of environmental pollution, etc. are intended, including combustion controls for automobile engines and detectors for partial or incomplete combustion in domestic gas water heaters.
In the oxygen sensors to which the electromotive function of solid electrolytes, for instance, ZrO.sub.2 -base solid electrolytes are formed into a plate-like shape, and are used in a state where ZrO.sub.2 is partially stabilized by the addition of small amounts of stabilizers such as Y.sub.2 O.sub.3. For instance, the amount of Y.sub.2 O.sub.3 to be added has been usually about 4.5 mol % to maintain the mechanical strength at a given value or higher.
However, if it is intended to increase the mechanical strength by reducing the amount of Y.sub.2 O.sub.3, a bending strength reaches as high as 50 kg/mm.sup.2 or more, but there is a drop of electrical conductivity so that it insufficiently functions as the resistor of the ZrO.sub.2 element. On the contrary, the addition of Y.sub.2 O.sub.3 in an amount exceeding 6 mol % results in improvements in electrical conductivity, but causes a reduction of a bending strength to 30 kg/mm.sup.2 or less.
Generally, the sensing element of oxygen sensors is constructed by attaching platinum electrodes to both sides of a wholly or partially stabilized zirconia solid electrolyte-base thin plate. However, since it has a thickness on the order of about several millimeters, it is very likely that it may be easily broken during processing or assembling, or in use. Thus, there is a demand for the development of detection elements excelling in mechanical strength.
Besides, in general, one means for enhancing the strength of sintered bodies involves the addition of the predetermined amount of impurities to the base material, whereby the growth of grains during sintering is inhibited. Another means for strengthening sintered bodies involves the coating of particles on the surface of the formed base material mass, said particles having an ion radius larger than that of said base material mass, thereby to cause substitution and solid-solution formation for producing compression stress in the surface of said base material mass.
According to the reinforcing manner in which the aforesaid impurities are added, however, the strength of the base material may be affected depending upon the type of said impurities. This needs time-consuming examination of the type of impurities. Thus, such a manner for reinforcing sintered bodies cannot be said to be a generally applicable reinforcing treatment without difficulty. According to another reinforcing manner to form the coating layers, the compression stress varies depending upon the type of the coating materials, viz, the degree of substitution and solid solution formation with respect to the base material, with the resulting variations in strength. This leads to the deficiency of reinforced stability and, in some cases, may result in the formation of cracks on the surface of sintered bodies.
Furthermore, there are oxygen sensors which is constructed by laminating Al.sub.2 O.sub.3 layer on the surface of zirconia solid electrolyte substrate, such as U.S. Pat. No. 4,502,939 or U.S. Pat. No. 4,334,974. However, those solid electrolyte function elements have still not satisfactory strength.